Fracture by trauma is the common disease in orthopaedics, and the treatment of the unstable fracture (comminuted fracture) is a longstanding problem for the surgeons. At present, the methods of incision restoration with internal fixation have been widely adopted in clinics home and abroad, and the main of which include micro-plates screw fixation, intramedullary fixation, tension wire fixation, introosseous wire suture fixation, intraosseous nylon suture fixation, traditional intersection Kernig needle fixation and absorbable polymeric screw fixation, and so on. These methods have different flaws, which affect the therapeutic effect and functional restoration of the patients. For example, since all of the internal fixation materials are foreign ones, the foreign reactions inevitably occur with different extents. So the second operation is unavoidable in order to take the internal fixation materials out, which certainly brings the patient extra pain and economic burden. The large area of the wound affects the healing of the bone. The operation is a troublesome process with high degree of difficulty. Moreover, the biocompatibility of the implanted materials is bad and the fixed strength is low, especially in the spongy bone areas.
In order to overcome the shortcomings of internal fixation and to solve the problems on the fixation of the small blocks of comminuted fractures, the methods taken in clinics nowadays include the follows:
1. Polymethylmethacrylate (PMMA) Bone Cement
Polymethylmethacrylate (PMMA) bone cement is composed of methylmethacrylate, initiator and some filling materials. The free radical polymerization occurs following the addition of monomer liquid in powder until turning into hard solid. Before setting, the paste can be easily molded and show adhesion. It is usually used for the fixation of artificial joint prosthesis and that of some comminuted fracture (Kerong Dai, Bone and Joint Injuries Magazine, 1995, 10(4): 210–212). But the strong exothermic behavior in the setting reaction will cause the peripheral tissues necrosis. As the main components of organic glasses, PMMA is aged, loosing and falling apart after a long time implantation due to its bad biocompatibility. Its monomer is poisonous and has stimulating smell, which can bring the blood pressure of the patients lower suddenly and even result in the sudden death. When it is used as adhesion, the ratio of long-dated loose and revision of prosthesis is relatively high. The non-degradation characteristic of the materials will keep the fracture from healing and growing for comminuted fracture. In general, the effect of the fixation in comminuted fracture is not very ideal due to the defects of the material itself.
2. For comminuted fracture, fracture reduction is usually adopted, followed by microplates screw fixation (Prevel et al, J-Hand-Surg-Am, 1995; 20(1): 44–49) or intramedullary fixation (Gonzalez et al, Clin-orthop, 1996, 327:47–54), which is effective for the massive bone fracture. It is much difficult to the fixation of the small blocks of bone fracture. In spongy bone areas, moreover, the screw fixation method still has the disadvantage of weak strength, even for the massive bone fracture. The postoperative slippage occurs frequently, which affects the postoperative effects and makes the secondary operation necessary.
Some absorbable internal fixation materials have been developed to eliminate the secondary operation, such as polycaprolactone (Lowry K. J et al., J. Biomed Mater. Res., 1997 36(4): 536–541), and calcium phosphate glass fibre enhanced poly-lactic acid (Slivka M. A et al., J. Biomed. Mater. Res., 36(4): 469–477). But the rapid decline in their mechanical strength limits their applications.
3. For screw fixation, the slippage usually occurs due to the incompact combination between the screw and the spongy bone. The polymer bone cement, such as PMMA, is used to perfuse the bore surrounding the screw in clinics and the screw is fixed after the setting of the cement. Consequently, the fixation strength is enhanced. But it becomes loose and falls apart eventually due to the bad biocompatibility and non-degradation characters of PMMA itself.
4. At present, PMMA is used popularly in the joint fixation and filling of defects after the revision of the prosthesis. (Kuhn K. D. et al., bone cements, Berlin: Springer-Verlag, 2000). Although some improvements on PMMA have been made including the decrease of its heat liberation, the increase of fluidity and injection capacity, some disadvantages still exist, like bad biocompatibility, long-term shedding and higher revision ratios.
For the fixation of the fracture, screw and prosthesis of the joints, all available materials and methods exist disadvantages with different extents. It is desirable to improve their properties, especially biocompatibility, exothermicity and fixation strength.
The magnesium phosphate cement (MPC) is popularly used in the rush repair of the airfield and road, owing to the rapid setting and high early strength characteristics (WO 9721639 AI, 1997). Weill et al., disclosed a kind of calcium phosphate bone cement containing magnesium oxide, soluble phosphate, sand and fly ash, which is used in the rush repairs in constructions (U.S. Pat. No. 4,756,762). Sechra disclosed a fast setting cement used in concrete pavements repairs (Cement Concrete Research 1993, 23: 254–66). All of these have no special requirements to the components and purities of the raw materials, hydration reaction heat, and the toxicity of the materials and additives.
Hirano et al., disclosed the calcium-containing magnesium phosphate cement with the component of Ca3Mg3(PO4)4 and eugenol solvents, which is used for the root canal filling and repair (JP 04352706, 1992). The results indicated that the cement was non-toxic with good biocompatibility. But its hardening process was slow and lasts about 40 min. In particular, the early strength is not high.
Frazier D. D et al, disclosed a kind of poly (propylene glycol-fumarate) bone cement reinforced by particles of calcium carbonate or calcium phosphate. Its early adhesive strength can up to 30 MPa, and its highest compressive strength can reach 300 MPa (J. Biomed. Mater. Res., 1997, 35(3): 383–389). Sakai T. et al., disclosed 4-META/MMA-TBB bone cement filled with hydroxyapatite (HAP) (J. Biomed. Mater. Res., 2000, 52(1): 24–29), which indicated that the incorporation of HAP was beneficial for the post-stability of the cement and bone fixation. But the compressive strengths of these two kinds of cements were derived from the polymerization of monomers, rather than the hydration of the inorganic component. Moreover, the materials can't be absorbed after being implanted in vivo, and the foreign materials exist all along. In addition, the heat liberation from polymerization process will bum the peripheral tissues.